1. Field of the Invention
This invention is related to the field of measuring material sample characteristics by the interaction of energy with matter. More specifically, the invention concerns measuring and/or determining characteristics of a material by analyzing the characteristics of an energy beam incident upon and affected by the sample. In the described embodiments, polarized radiant energy reflected and/or transmitted by the material sample is analyzed to infer optical properties such as index of refraction, and physical properties such as material thickness.
In this description, the term "thin film" will be used to define any material whose thickness is to be measured. For ease in describing the invention, however, hereinafter the term will refer to a material that can be deposited as a thin film by a variety of processes such as chemical, mechanical, or high-vacuum evaporation techniques. Typically, such thicknesses are in the vicinity of near zero to 20,000 angstroms. Since film thickness test results using radiant energy beams must depend, at least in part, upon the energy beam traversing through the body of the material under test, the material to be tested must be either transparent transluscent or at least partially transmissive to the impinging energy beam.
2. Brief Description of the Prior Art
In addition to the obvious mechanical devices used for measuring the thickness of materials (considered rather thick in comparison to the types of materials to be measured using the devices and methods of the present invention), a variety of known techniques and devices have been conceived for measuring the thickness of a thin film material using radiant energy beams, and in particular, laser beams.
One such method employs the use of an ellipsometer, that is, a spectrometer equipped with polarizing prisms and retardation plates, used primarily in the analysis of elliptically polarized light in the study of thin evaporated films. In ellipsometry, a beam of collimated light is directed through a polarizer and subsequently through a quarter-wave retardation plate, after which it is reflected by the thin film under test. This reflected beam is then directed through a second polarizer which is generally called an analyzer. Two of the three optical elements are then interactively rotated so as to produce extinction of the beam exiting the second polarizer. The angular positions of the elements are then accurately measured and used to solve for thin film thickness.
As will be discussed in detail later, incident light impinging upon the surface of a thin film sample at a given angle will be reflected in different amplitudes, depending upon the angle of polarization. Accordingly, by knowing the incident angle, the index of refraction of the material, and the amplitudes of the two coherent plane-polarized elements of the incident beam, sufficient information from a detector detecting the amount of reflected light attributed to each of the two beam elements can be derived to determine the thickness of the film. While ellipsometry has been satisfactory for many applications in determining thickness of thin film materials, this method of measurement uses mechanical rotation of optical elements to find a power null and requires precision angular measurement of the angles of rotation. While these are not great disadvantages in slow manual operation where the operator rotates the elements by hand while seeking a null and then takes readings from angle verniers, they are expensive to automate. To automate such an ellipsometer would require servos to rotate the elements interactively to find a null, as well as precision angular encoders to determine the precise angles needed for accurate measurement.
A second method of determining thicknesses for thin films is by using a variable frequency or variable wavelength light source, again directing the light beam from the source at a given precise angle relative to the surface of the sample under test. By knowing the index of refraction for the material and noting the angle of incidence of the beam, and by determining the precise wavelength of the incident light at which a maximum amount of reflected light is detected, the thickness of the sample can be determined. Incremental errors having a magnitude of some multiple of 1/2 wavelength of the incident light must be accounted for in the final results. This is due to the fact that the light traversing the thickness of the film and combining with the reflected light from the upper surface thereof reinforces the latter to a maximum degree when the distance traveled through the film is equal to a full wavelength of the incident light, and cancels the reflected light to a maximum degree when the distance travelled is one-half wavelength. Variations between minimum (maximum cancellation) and maximum are cyclically repeated as a function of change of distance travelled through the sample so that an intensity measurement of a particular magnitude of reflected light will occur twice each cycle, and, of course, there may be more than one cycle of phase difference between the beam portions reflected from the top and bottom surfaces of the sample. This 1/2 wavelength increment error is inherent in all thin film measuring schemes using reflected light beams. The problem is greatly magnified in the variable frequency technique, however, since the wavelength is varying and thus adds another variable to the already complex formula for calculating thickness, and some additional information must be fed into the computation before reliable results can be obtained. Again, although effective for measuring small thicknesses of thin film material, a number of disadvantages of this technique can be readily appreciated. First, when a peak is found in the reflected beam, the exact wavelength of the varying frequency light source must be determined with expensive and precision instrumentation. Moreover, a plotted curve for the relationship of reflected intensity versus wavelength will show that the knee of the curve is rather broad, having a peak with a slowly changing amplitude making the selection of the actual peak point on the curve indeterminate. As a result, an inherent tolerance figure must be accounted for when the determination is made that a peak in reflected intensity level has been detected. Another critical factor in establishing the credibility of a thickness measurement with this technique is the fact that the index of refraction for the particular material being measured must also be known at the exact frequency of the incident light which produced the peak in reflected energy. This determination is nesessitated by the dispersion parameter of the material under test, i.e., the change of index of refraction with change of wavelength of incident light. While a number of machines have been developed to measure the dispersion of the film the need for inputting the dispersion factor into the formulas for calculating the film thickness adds significantly to the cost of the procedure and renders the procedure impractical for nominal users. Moreover, the exact dispersion factor for a given material changes with deposition process variations. Accordingly, where dispersion factors enter into measurement analyses, only approximate or assumed values can be used.
A third, and more simplified, method for measuring film thickness involves the impinging of a single wavelength light beam onto a surface of a sample and measuring both the intensity of the incident beam and that of the reflected beam. Again, with the known angle of incidence, the known index of refraction of the material, and the precise wavelength of the incident beam, the ratio of intensity of reflected beam to that of incident beam gives the amount of loss due, for a clean sample, to the interference effects of the beam elements reflected from the upper and lower surfaces of the sample. Thus, by knowing the amount of cancellation or reinforcement, the path length of the incident beam through the material in terms of incident beam wavelength can be determined, and through mathematical calculation, the thickness of the film can be determined. One of the major disadvantages of this type of simplified system is the fact that the intensity level of the reflected beam is not necessarily due in total to the light wave interference effects. Any defects in the material at the point of impingement of the beam, such as scratches, dust particles, surface irregularities, and the like, will account for some percentage of the loss of light from source to detector. Accordingly, several regions on the sample must be measured, and until consistency of results are determined, the true thickness of the film cannot be ascertained. Since some surface defects can occupy relatively large regions for such samples as integrated circuits, or even span the entire surface (as in a mottled surface), there may not be sufficient surface area available to permit the number of measurements needed for eliminating the contribution of defects from the final results.
In the aforementioned prior art system, one must either (1) produce and measure very precise mechanical rotations, or (2) measure the absolute quantities of light striking photodetectors with an essentially defect-free optical path and thin film sample, or (3) determine the peak value of a slowly varying light level and know the precise wavelength at that peak and also assume an index at that wavelength because of dispersion.
Accurate film thickness test results thus depend upon absolute measurements of the amount of light detected, and drift of system parameters, for example temperature, power supply voltages, surface blemishes, detector electrical and mechanical position drift, noise, and the like, all contribute to thickness calculation errors, because the detector output has no information in it to discern the difference between loss of light energy due to the wave interference effects and that due to misadjusted, defect-altered, or drifting parameters.